1. Field of the Invention:
The present invention relates to an electromagnetic wave shield type circuit unit, and particularly to an electromagnetic wave shield type circuit unit capable of preventing the external leakage of electromagnetic wave from a circuit substrate mounted on an electromagnetic wave shield casing and an improved TV game playing machine circuit unit utilizing the same.
2. Description of the Related Art:
With the advance of electronic technique, substrates on which electronic circuits and the like are mounted are increasingly used for various modern electronic/electric instruments, particularly for TV game playing machines.
However, noise is frequently created by electromagnetic waves leaked from various components in such electronic circuits on the circuit substrates. Such noise may interfere with the operation of the other electronic instrument located adjacent to such an electronic instrument, leading to malfunction.
In order to avoid such noise, there have been broadly used circuit units which use electromagnetic wave shield casings. Typically, such a circuit unit includes a circuit substrate fixedly mounted in the shield casing and adapted to prevent the electromagnetic waves from leaking out of the casing.
However, such a circuit unit requires a slitted window formed in the shield casing, through which the end of the circuit substrate having external connection terminal means can extend outwardly. This means that electromagnetic waves leak externally through the slitted window in the shield casing.
FIGS. 9 and 10 in the accompanying drawings show an example of the conventional circuit units used in the prior art, as described in Japanese Utility Model Publication No. 3356/1982.
In these figures, reference numeral 1 denotes a shield casing of electrically conductive material, which has a slitted window 2 formed therein. The casing 1 receives a circuit substrate A through the slitted window 2. The shield casing 1 comprises a main body 1' and a lid 1", which are detachably connected with each other to form the interior of the casing. The circuit substrate A is fixedly mounted in the lid 1". Thus, the circuit substrate 1' can be removed out of the casing together with the lid 1" when the latter is demounted on the main body 1'.
The shielding effect of such a shield casing 1 is generally represented by: ##EQU1## where the reflection loss is one provided by the reflection of a part of electromagnetic wave off the boundary between the shield casing and a medium which has a different surge impedance and is placed in the path of propagation of the electromagnetic wave; the absorption loss is one provided by the attenuation of the electromagnetic wave entering and propagating the material of the shield casing, the absorption loss being consumed by current induced in the shield as Joule heat; and the multi-reflection loss is one created by reflections on the opposite boundaries of the material of the shield casing, the multi-reflection loss been capable of being neglected if the absorption loss is sufficiently large.
The aforementioned losses are determined from Schlutz's formula. However, the Schlutz's formula must be used on the premise that the shield member is infinitely planar without seam. Actually, thus, many correction values are required when the Schlutz's formula is used. Most of the correction values concern the shielding material and there are also correction values relating to the seam in the shielding member.
Thus, the shielding effect depends on the handling of seam even if shield casings having the same configuration are made of completely the same material. In the conventional circuit unit shown in FIGS. 9 and 10, which includes the slitted window 2 for externally exposing the end of the circuit substrate A, the magnitude of such correction values becomes increased. There is therefore a problem in that they cannot perform their shielding effects sufficiently.
This is because the slitted window 2 cannot shield electromagnetic waves having wavelength less than two times the length of the window. If electromagnetic waves having such wavelength enter the window 2, they will easily pass through the slitted window 2 to leak out of the shield casing. In the modern electronic instruments, particularly, the frequency of clocks used therein progressively increases. Another problem is frequently raised that electromagnetic waves of high frequency created from the circuit substrate A leak out of the shield casing directly through the window 2 thereof.
As shown in FIG. 11, the conventional circuit units may have a window 2 formed to have its width larger than the thickness of the circuit substrate A. In such case, more noise may leak out of the shield casing through the window 2.
In the conventional circuit units, furthermore, electric currents induced by the electromagnetic waves from the circuit substrate A act on the edges of said window 2 which in turn act as an antenna for re-radiating the noise externally. This phenomenon is called as "edge effect". This is because as the frequency of the electromagnetic waves increases, the cut end faces of the electrically conductive member act as a micro-split antenna.
From the foregoing, it is to be understood that the conventional circuit units will not provide any satisfactory shielding effect since they have relatively large correction values.